1. Field of the Invention
The present invention relates to a driving method and a driving device for a display device such as an electroluminescence (abbreviated as EL) display device or other AC driven capacitive flat matrix display panel (hereinafter referred to as the "thin-film EL display device").
2. Description of the Prior Art
For example, a double insulation (or three-layered) thin-film EL element is constructed in the following manner.
As shown in FIG. 1, strips of transparent electrodes 2 composed of In.sub.2 O.sub.3 are formed parallel to one another on a glass substrate 1. Then, a dielectric layer 3 composed of Y.sub.2 O.sub.3, Si.sub.3 N.sub.4, TiO.sub.3, or Al.sub.2 O.sub.3, an EL layer 4 composed of ZnS or other material doped with an activating agent such as Mn, and another dielectric layer 3a composed of Y.sub.2 O.sub.3, Si.sub.3 N.sub.4, TiO.sub.3, or Al.sub.2 O.sub.3, each layer having a thickness of 500 to 1,000 .ANG., are deposited one on top of another above the transparent electrodes 2 using a thin-film technique such as evaporation or sputtering, to form a three-layered construction. Finally, strips of counter electrodes 5 composed of Al are formed parallel to one another, and at right angles to the transparent electrodes 2, on top of the three-layered construction.
The thus constructed thin-film EL element can be considered as a capacitive element in terms of circuit equivalence since the EL layer 4 sandwiched between the dielectric layers 3 and 3a is interposed between the electrodes. As is obvious from the voltage-to-brightness characteristic curve indicated by the reference sign a0 in FIG. 2, the thin-film EL element is driven by a relatively high voltage on the order of 200 V.
In a thin-film EL display device using the above thin-film EL element for a display panel, either the transparent electrodes 2 or the counter electrodes 5 of the thin-film EL element are configured as the scanning-side electrodes and the other as the data-side electrodes, the driving of the display being performed in such a way that a write voltage is applied to the line-sequentially selected scanning-side electrodes by a scanning-side driving circuit consisting of n-channel high voltage MOS (Metal Oxide Semiconductor) driver ICs (integrated circuits) and p-channel high voltage MOS driver ICs, while applying a modulated voltage, which determines emission or non-emission of light according to the display data, to the data-side electrodes by a data-side driving circuit consisting of n-channel high voltage MOS driver ICs and p-channel high voltage MOS driver ICs.
For driving the display, considering that the thin-film EL element is a capacitive element, an AC driving method is employed in which the p-channel driving to apply a write voltage of positive polarity and the n-channel driving to apply a write voltage of negative polarity to the scanning-side electrodes with respect to the ground potential such as the chassis are alternately performed for every frame.
FIG. 3 is a circuit diagram showing an example of the configuration of a thin-film display device in which the prior art driving method is employed. In the display device shown, a thin-film EL element having an emitting threshold voltage Vw (=190 V) is used for a display panel 10, in which the data-side electrodes are arranged in the X direction and the scanning-side electrodes are arranged in the Y direction.
Scanning-side n-channel high voltage MOS driver ICs 20, 30 and scanning-side p-channel high voltage MOS driver ICs 40, 50 are the circuits constituting the above-mentioned scanning-side driving circuit.
On the other hand, a data-side driver IC 160 is the circuit that constitutes the above-mentioned data-side driving circuit.
A source potential switching circuit 80A is a circuit for switching the source potential for all p-channel MOS transistors PT1-PTi in the p-channel high voltage MOS driver ICs 40, 50 between a positive-polarity write voltage Vw+1/2Vm (=220 V) and a voltage of 0 V.
A source potential switching circuit 90A is a circuit for switching the source potential for all n-channel MOS transistors NT1-NTi in the n-channel high voltage MOS driver ICs 20, 30 between a negative-polarity write voltage-(Vw-1/2Vm) (=-160 V) and a voltage of 0 V.
A data inverting circuit 100 is a circuit comprising an exclusive OR gate, etc. for inverting a data signal DATA, which is input to the data-side driver IC 160, in response to a control signal RVC1.
According to the prior art driving method employed in the above thin-film EL display device, a voltage Vw (=+190 V) approximately equal to the emitting threshold voltage is added to a constant voltage 1/2Vm to provide a voltage of Vw+1/2Vm as the write voltage for the p-channel driving, while a negative-polarity constant voltage-(Vw-1/2Vm) is provided as the write voltage for the n-channel driving. In this case, the voltage Vm is set at 60 V. Therefore, the write voltage for the p-channel driving is given by: EQU Vw+1/2Vm=190V+1/2.times.60V=220V (1)
and the write voltage for the n-channel driving is given by: EQU -(Vw-1/2Vm)=-190V+1/2.times.60=-160V (2)
As regards the modulated voltage, 0 V is set for the p-channel driving, and Vm (=60 V) for the n-channel driving, as the modulated voltage for light emission, while Vm (=60 V) is set for the p-channel driving, and 0 V for the n-channel driving, as the modulated voltage for non-emission of light. Therefore, the voltage given by the following equation is applied to the picture elements for light emission during the p-channel driving, with the potential of the scanning-side electrodes as the reference. EQU Vw+1/2Vm-0V=190V+1/2.times.60V=220V (3)
The voltage applied during the n-channel driving is given by: EQU -(Vw-1/2Vm)-Vm=-(Vw+1/2Vm)=-(190V+30V)=-220V (4)
On the other hand, the voltage given by the following equation is applied for non-emission of light during the p-channel driving. EQU Vw+1/2Vm-Vm=Vw-1/2Vm=190-30V=160V (5)
The voltage applied during the n-channel driving is given by: EQU -(Vw-1/2Vm)-0V=-(Vw-1/2Vm)=-(190V-30V)=-160V (6)
In FIG. 2, the voltages Vw, (Vw+1/2Vm), and (Vw-1/2Vm) are marked alongside of the corresponding voltage values on the x-axis along which the applied voltage is plotted.
Referring to FIG. 3, the display device shown is driven in response to externally supplied two synchronizing signals, a vertical synchronizing signal V and a horizontal synchronizing signal H. That is, the p-channel or the n-channel driving is performed on the scanning-side electrodes in line sequential fashion in synchronism with the horizontal synchronizing signal H, and one frame is formed when the line sequential driving of all the scanning electrodes is completed. The vertical synchronizing signal V usually indicates the beginning of one frame, and in synchronism with this signal, the driving for one frame is initiated. Each scanning-side electrode is subjected in line sequential fashion to the p-channel or the n-channel driving once during one frame period. The EL display element 10 requires the applied voltage to alternate, and the p-channel driving and the n-channel driving are performed alternately in such a way that the alternating current cycle is closed with two frames for every scanning-side electrode. To achieve this, there have been proposed a method (field inversion driving) in which all scanning-side electrodes are subjected in line sequential fashion to driving of one polarity in one frame period, and a method (line inversion driving) in which the driving is performed by inverting the polarity for every scanning line.
However, since the thin-film EL element is a capacitive element as described earlier, a modulating voltage of OV or Vm according to the display data is applied to all the data-side electrodes X, repeating charge and discharge every time each scanning-side electrode Y is selectively driven in line sequential fashion; therefore, the thin-film EL element has the problem of consuming a large amount of modulation power.
Generally, in a capacitive element of capacitance C, when charge and discharge is repeated f times per unit time with a charge voltage V, the power consumption P is given by: EQU P=f.multidot.C.multidot.V.sup.2 ( 7)
Therefore, in the above-described prior art driving method, when the capacitance of the display panel 10 is denoted as C and the number of times to apply the modulated voltage Vm to the data-side electrodes X per unit time is denoted as f, the power consumption by the charging and discharging of the modulated voltage Vm, i.e. the modulation power Pm, is given by: EQU Pm=f.multidot.C.multidot.Vm.sup.2 ( 8)
Aiming at overcoming this problem, there have been proposed various driving methods including a step driving method in which the magnitude (voltage level) of the modulated voltage Vm is increased stepwise, but any of the thus far proposed methods has not been able to sufficiently reduce the modulation power.
When a write voltage is being applied to a certain scanning-side electrode Y, for example, Y1, the transistors PT2-PT1 and NT2-NTi for the remaining scanning-side electrodes Y2-Yi are turned off, and consequently, the remaining scanning-side electrodes Y2-Yi are placed in the so-called floating state. Therefore, the display power P associated with the capacitance C at the scanning-side electrode Y1 to which the write voltage is applied is relatively small. On the other hand, the modulated voltage 0 V or Vm is constantly applied to the data-side electrodes X, as described earlier, resulting in a large modulation power Pm because of the capacitance C between all the data-side electrodes X and all the scanning-side electrodes Y. Therefore, reduction in the modulation power Pm is effective for reducing the display power for the entire display device.